This invention relates to apparatus and methods of broadband amplification with high linearity and low power consumption.
Broadband signal amplifiers and methods for amplifying signals over a wide range of frequencies have many applications. For example, wireless communication devices, such as cellular telephones, cordless telephones, pagers, television tuners, local area networks, and the like, require wide dynamic range, high linearity amplifiers to boost the strength of received signals without distortion and cross-talk between different frequencies. Unfortunately, all linear amplifiers distort the signals they are required to amplify to some degree. This is particularly undesirable when two or more independent channels are being amplified. Under these circumstances, the amplifier generates unwanted intermodulation products that may cause interference and result in poor performance of the communication device.
Intermodulation distortion is defined in terms of the peak spurious level generated by two or more tones injected into a receiver. A receiver may be characterized by a third-order distortion figure of merit referred to as a "third-order input intercept point" (IIP3), which is defined as the input power (in the form of two tones) required to create third-order distortion products equal to the input two tone power. The linearity of an amplifier, and thus the immunity of the receiver to distortion and cross-talk, improves with the IIP3 of the amplifier. Another amplifier figure of merit is equal to the ratio of the maximum microwave output power (at a specified level of distortion) to the applied DC power. The conventional method of specifying the level of distortion for this figure of merit is called the output intercept point of third order products (OIP3). In accordance with the OIP3 method, two input signals separated only slightly in frequency, and of substantially equal, but adjustable, power are applied to the amplifier input. A plot is made of both the fundamental frequency output power and the power in the third order intermodulation product versus the input power and a linear extrapolation is made of these two plots. The point where these two extrapolations intersect is the OIP3 amplitude, which is read in dB from the output power (ordinate) axis.
Amplifier linearity may be improved by increasing amplifier gain (see, e.g., Wheatley, U.S. Pat. No. 5,732,341). This approach, however, increases the power consumption of the amplifier and, thereby, decreases the length of time a user may operate battery-powered receivers incorporating such amplifiers before the batteries must be recharged.